1. Field of the Invention
The present invention relates to a loading pressure compensation type logic valve to control the flow rate of a hydraulic pressure control valve which is used, for example, in construction machines.
2. Description of the Prior Art
The applicant of the present invention has presented loading pressure compensation type logic valves, which are the prior art of the present invention. Japanese Patent Publication No. 163503/1990 and the specification and the drawings of Japanese Utility Model Application No. 89878/1989 disclose such devices. These loading pressure compensation type logic valves are capable of compensating for the effects of loading pressure on stroke control.
The pressure compensation type logic valve described in the specification and the drawings of Japanese Utility Model Application No. 89878/1989 is explained hereunder, referring to FIG. 2.
The stroke length of a valve poppet 44 which has a flow rate control orifice group 74 is determined by a balance between loading pressure P.sub.IN at a loading pressure inlet port 96, a pressure in a poppet spring chamber 55, the opposed pressure-receiving areas (both portions have identical pressure-receiving areas) and a spring force of a poppet spring 56. It is possible to linearly control the difference between loading pressure P.sub.IN at loading pressure inlet port 96 and the pressure in poppet spring chamber 55, which determines the aforementioned balance, using an outside pilot pressure P.sub.i and such equipment as spools 46 and 47.
More precisely, loading pressure P.sub.IN at loading pressure inlet port 96 is directed to pressure chambers 63 and 64 through orifices 77 and 83 respectively (pressure P.sub.64 in pressure chamber 64 is virtually identical to loading pressure P.sub.IN) and brought into equilibrium by forces exerted on pressure receiving areas A.sub.63 and A.sub.47 (A.sub.63 =A.sub.47) at the ends of spools 46 and 47. The ends of spools 46 and 47 respectively facing pressure chambers 63 and 64 have identical diameters.
Outside pilot pressure P.sub.i applied to an outside pilot pressure inlet port 97 is directed into a stepped pressure chamber 66 (its pressure receiving area is represented by A.sub.66) of spool 46. As shown in the following equation, pressure P.sub.63 in pressure chamber 63, which is equal to pressure P.sub.55 in poppet spring chamber 55, can be controlled by using the pressure described above and a spring force F.sub.70 of a spool spring 70. EQU P.sub.64 .multidot.A.sub.47 +F.sub.70 =P.sub.i .multidot.A.sub.66 +P.sub.63 .multidot.A.sub.63.
Therefore, EQU P.sub.IN .multidot.A.sub.63 +F.sub.70 =Pi.multidot.A.sub.66 +P.sub.55 .multidot.A.sub.63.
Consequently, differential pressure .DELTA.P between loading pressure P.sub.IN at loading pressure inlet port 96 and pressure P.sub.55 in poppet spring chamber 55, which determines the aforementioned balance, is: EQU .DELTA.P=P.sub.IN -P.sub.55 =(P.sub.i .multidot.A.sub.66 -F.sub.70)/A.sub.63
which is a linear function of outside pilot pressure P.sub.i and therefore not affected by loading pressure P.sub.IN.
As described above, as far as stroke control is concerned, a valve according to the prior art is not influenced by loading pressure P.sub.IN. However, flow from loading pressure inlet port 96 through flow rate control orifice group 74, flow rate control chamber 54 and poppet seat 52 into drain chamber 53 is conrolled only by the degree of aperture of flow rate control orifice group 74. In other words, the apparatus of FIG. 2 lacks a mechanism to maintain a differential pressure between the front side and the rear side of orifice group 74, i.e. the differential pressure between loading pressure inlet port 96 and flow rate control chamber 54. As a consequence, when controlling a great quantity of fluid, a valve according to the prior art functions only as a conventional throttle valve.
Further, as shown in FIG. 3, it is possible to make a flow rate control mechanism that is free from the effect of loading pressure by installing flow control valve 95 between valve poppet 44 and loading pressure inlet port 96 of a logic valve otherwise the same as shown in FIG. 2. However, such a structure presents the problem that the two-part valve structure increases the bulk of the valve and adds leaks Q.sub.3 and Q.sub.4 at flow control valve 95 to leaks Q.sub.1 and Q.sub.2 already existing in the logic valve of FIG. 2.